On-site production of prescription lenses is currently on the rise. Advances have recently been made on two fronts in materials and methods for the on-site production of prescription lenses: advances designed to reduce the amount of stock materials which must be carried on site, and advances in reduction of the time needed to produce the prescription lenses. However, none of these approaches has made it possible to eliminate the need to carry wafer lens for adhesion to base lens. Further, no approach has made it possible to transform stock prescription lenses to lenses of different prescriptions or to bifocals or multi-focal lenses in a matter of minutes, including the option of rendering the lenses photochromic. Further, none of the available methods permits the lens crafter to utilize a flexible mold and monomer to form a simple wafer lens onto stock lenses during a curing process, yet offer a wide variety of photochromic properties, tinting, UV protection, scratch resistance, and other desirable properties.
More specifically, one approach to on-site custom lens production involves casting an additional plastic layer onto a plastic lens blank. See, for example, in U.S. Pat. No. 2,339,433 (Staehle) disclosing a method of adding a correction to a molded plastic lens by adding a thin level of resin. U.S. Pat. No. 3,248,460 also discloses means for casting plastic lenses from thermosetting or thermoplastic materials wherein a plastic blank having significantly less curvature than required for the full-intended prescription of the lens is used as a base. An additional layer of material is cast onto this base. The patent employs a conventional optical gasket to provide space between the plastic blank and the mold and to hold the resin material in the cavity created thereby. The additional layer of material changes the curvature of the resulting lens over the vast majority of its surface, thereby changing the prescription of the resulting finished lens to the power required. A disadvantage of this system is that the material must be cured by heat. Such a heat curing process requires heating over a period of more than 12 hours, thus making the formation of the lens a long, drawn-out process. A further obvious disadvantage is that photochromic lenses could not be produced from such materials and methods. Others have tried to manufacture multi-focal or progressive plastic lenses using a lamination technique. Such a technique joins a preformed plastic section, generally referred to as a wafer or wafer lens or veneer lens, to another cured plastic prescription lens. In all cases, the curvature of the wafer lens must correspond to the mating curvature of the base lens, and, thus, the number of wafer lenses which must be maintained in stock corresponds to (a) the number of base lens curvatures to be matched times, (b) the strengths of the corrections times, and (c) the various strengths of progressive or bifocal prescriptions [i.e., (a) x (b) x (c)]. A base lens is matched to a preformed wafer section defining a multi-focal or progressive region and the conforming mating surfaces of these lenses are joined by an adhesive. See, for example, U.S. Pat. No. 4,940,205 (Rudd, et al.) teaching a method and apparatus for forming a laminated bifocal lens which includes selecting a frontal lens component having a bifocal element and bonding the frontal lens component to a rear lens component. The non-prescription frontal lens carrying the bifocal element is referred to as a veneer lens, and the rear lens is referred to as the prescription lens. The arced inner surface of the veneer lens meniscus has the same curvature as the front surface of the prescription lens. To fill a prescription, a technician matches the desired frontal lens with a rear lens carrying the desired prescription base. The adhesive for bonding the two lens components is the same monomer used to cast the lens components. Accordingly, the curvature of the veneer lens must match that of the prescription lens, and no provision is made for incorporation of photochromic pigments.
As a somewhat different approach, see also U.S. Pat. No. 5,433,810 (Abrams) teaching a method and apparatus for lamination of composite eyeglass lenses. Front and rear lenses are laminated together by holding the front lens in an XY adjustable stage on a laminating axis, holding the rear lens in a pre-determined position relative to the laminating axis, and moving the two lenses together on the laminating axis to spread an adhesive between them. Then, the adhesive between the pressed-together lenses is cured by UV radiation directed through the front lens before the lenses are removed from the laminator. The bonding process involves placing a curable adhesive on the concave interface surface of the front lens; pressing the convex interface surface of the rear lens against the adhesive in the front lens to spread the adhesive throughout the space between the two lenses; and curing the adhesive to bond the lenses together, forming a composite lens. The curvatures of the mating surfaces conform to each other, but the convex surface may have a slightly greater curvature, up to 0.25 diopter greater, in order to facilitate the spread of adhesive between the two lenses from the inside out (col. 3, line 34). While Abrams teaches an apparatus for aligning and laminating front and rear lenses to form a composite eyeglass lens, Abrams does not teach lens compositions and does not address photochromic lenses.
Photochromic ophthalmic lenses made of mineral glass are well known. Photochromic pigments have good compatibility with mineral glass. However, photochromic mineral glass lenses are heavy and have a slow photochromic reaction time, particularly in the change from dark lenses to light lenses. Today, however, most spectacle lenses are made from any of a variety of plastics or from plastic-glass composites. Plastics include acrylic, PPMA (a product of PPG-Pittsburgh Plate Glass) also known as CR-39, and LEXAN (Polycarbonate made by General Electric). For example, U.S. Pat. No. 3,946,982 (Calkins) discloses a method of casting multi-focal lenses. He does this by holding two mold portions together with a gasket, one of the molds having a recessed portion, so as to provide a bifocal effect. Liquid plastic is injected between the two mold portions, cured and cooled, thereby creating an entirely new lens. However, this is a slow, laborious and time-consuming process.
Recently, attempts have been made to apply photochromic pigments to lightweight plastic lenses to render them similarly photochromic. However, for various reasons, this objective has not been satisfactorily achieved with plastic lenses. One reason for the lack of success has to do with the chemistry of ethylene glycol diallyl dicarbonate, the most commonly used monomer for producing plastic ophthalmic lenses. This monomer is cast in a lens mold and polymerized with a catalyst such as isopropyl percarbonate. One might expect that a plastic lens made from such a monomer could be rendered photochromic simply by incorporating photochromic pigments into the monomer composition prior to casting the lens. However, in practice it was found that, following polymerization of the organic material, the photochromic pigments did not retain their photochromic property.
Apparently, the catalyst required for the polymerization caused inhibition of the pigments. Thus, it has not been possible in practice to simply incorporate photochromic pigments into the monomer composition when making ophthalmic lenses from an organic material.
One type of approach to rendering plastic ophthalmic lenses photochromic required embedding a solid layer of photochromic mineral glass within the bulk of the organic lens material. For example, U.S. Pat. No. 5,232,637 (Dasher, et al.) teaches a method of producing a glass-plastic laminated ophthalmic lens structure. A thin, flexible, plastic adhesive layer is applied to a glass element and a monomeric formulation is flowed onto the adhesive layer and cured to form a laminated lens blank. The adhesive layer is a thermoplastic urethane that may be preformed by extrusion as a thin sheet. The sheet may be in the order of 0.13 to 0.63 mm thick, preferably 0.375 to 0.5 mm thick. The sheet may be, by way of example, an aliphatic polyether type urethane available from Thermedics under the designation SG-85A. Other thermoplastic resins such as polyvinyl butyral, 1,4-butane diol, polyetherpolyol and aliphatic diisocynate may be used. However, due to problems with glass-plastic adhesion, differences in the respective thermal coefficients of expansion, and due to contraction of organic materials during polymerization, such lenses exhibited stress fractures in production or could not stand up to extended normal handling by the consumer.
A variation on the glass-plastic composites is taught in U.S. Pat. No. 4,300,821 (Mignen). This patent teaches an ophthalmic lens made of organic material having at least one layer of photochromic mineral glass within its mass to impart photochromic properties to the lens. The photochromic mineral glass has a fibrous structure and may comprise a piece of woven fabric produced from fibers of photochromic mineral glass possessing a refractive index and coefficient of chromic dispersion which are equal to those of the constituent organic material of the lens. However, while such an approach may make it possible to mass-produce photochromic lenses, the approach can not be utilized for the small-scale customized production of prescription lenses on an as-needed basis. It would be cost prohibitive for a medium-sized operation to stock a large number of photochromic lenses of various prescriptions, in addition to the non-photochromic lenses, in order to be able produce photochromic lenses on demand.
Recently U.S. Pat. No. 5,462,698 (Kobayakawa, et al.) entitled "Photochromic Composition" addressed the problems associated with specific photochromic compounds which tend to be slow-acting or inactive when incorporated in plastic, and solved the problem by use of a resin compound having at least one epoxy group in the molecule as the resin for forming the photochromic lens. However, this solution to the problem has limitations and drawbacks. Kobayakawa, et al.) (a) is directed to forming a lens having photochromic compound dispersed throughout, (b) requires the presence of multiple types of photochromic compounds in combination, (c) requires the use of a polymerizable compound having at least one epoxy group to form the lens, (d) requires polymerization in a heat furnace, with polymerization taking from 2 to 40 hours, and (e) reports fading time to 1/2 density measured after exposure to 60 seconds averaging 3 minutes (Table 1). Kobayakawa, et al. thus uses specific materials and requires a long time to produce a slow acting lens.
More recently, U.S. Pat. No. 5,531,940 (Gupta et al.) teaches methods for making optical plastics lenses with photochromic additives. According to a first embodiment of the invention, a casting resin having a low cross link density comprising polymerizable components (preferably including up to 50 wt % bisallyl carbonate) and photochromic additives, wherein all polymerizable components have a functionality not greater than two, is arranged between a mold and a lens pre-form and then cured. However, upon polymerization, the resin has a low cross-link density and forms a soft matrix. This soft matrix is unsuitable as the outer layer for photochromic lenses. According to a second embodiment of the invention, the casting resin (but substantially free of photochromic additives) is arranged between a mold and a lens pre-form and then cured. The resin is then impregnated with photochromic additives. In a third embodiment, the layering resin containing a photochromic additive is provided on the surface of a mold and cured to a gel state. Then, a casting resin that is substantially free of photochromic additives is arranged between the coated mold and a lens pre-form and cured. According to a fourth embodiment, a casting resin that is substantially free of photochromic additives is provided on the surface of a mold and cured to a gel state. Then, a casting resin containing photochromic additives is arranged between the coated mold and a lens preform and cured. There is no discussion of photochromic rate of reversal, and the photochromic material is represented as being too soft to expose to the environment.
There is thus a need for a method for production of non-prescription or prescription ophthalmic lenses, which method would enable an optometrist to stock a small number of lenses, flexible molds, and glass molds, and allow custom production of plastic lenses of any required prescription including bifocal lenses, progressive lenses, and also capable of rendering the lenses photochromic, tinted, UV-protective, and/or scratch resistant as required by the customer.